The power consumption, speed, noise, and distortion characteristics of radio frequency (RF) transceivers are significantly affected by the performance of the RF power amplifiers and mixers used in the transmit signal path and the receive signal path. If an amplifier or mixer has good (or high) linearity, the amount of signal distortion introduced by amplification is minimized and relatively high signal quality may be attained. However, in order to maintain high linearity, an amplifier or mixer must be operated at power supply currents levels that are much higher that the average peak-to-peak signal current. This greatly increases the power consumption of the RF amplifier or mixer, resulting in a tradeoff between linear performance and power consumption.
A very common stage in RF amplifiers is the differential pair stage. FIG. 3 illustrates conventional differential pair amplifier stage 300 according to one embodiment of the prior art. Amplifier stage 300 comprises bias current transistor 305, transistors 310 and 315, resistors 320 and 325, coupling capacitors 330 and 335, and inductors 340 and 345. Transistors 305, 310, and 315 are n-p-n transistors. For the sake of simplicity in explaining the operation of the amplifier stage, DC biasing circuits are omitted from FIG. 3.
The illustrated configuration is know as a degenerated differential pair. Inductors 340 and 345 degenerate (reduce) the gain at VOUT of amplifier stage 300, but introduce negative feedback is in the emitters of transistors 310 and 315, thereby giving a more linear response. Amplifier stage 300 is a single-ended input because the input signal VIN is applied only to one input of the stage (i.e., coupling capacitor 330). The other input is grounded. The bias voltage, Vbias, establishes a bias current, Ibias, in transistor 305. Ibias is split between the two branches of amplifier stage 300. If VIN is zero, one half of Ibias, flows through the first branch, which comprises transistor 310 and resistor 320. The one half of Ibias flows through the second branch, which comprises transistor 315 and resistor 325.
When a differential amplifier with emitter (or source) degeneration is driven from a single-ended input source, the time-varying voltage at Va, roughly equal to one half of VIN, can produce unwanted modulation of the value of the tail current. The signal at Va leads to a reduction of the circuit linearity, an important parameter of RF transceiver designs.
Therefore, there is a need in the art for improved amplifiers and mixers for use in RF transceivers. In particular, there is a need for a degenerated differential pair amplifier stage that does not reduce linearity due to unwanted modulation in the value of the tail current.